1. Field of the Invention
The invention relates to a fuel cell, and more particularly to a fuel cell electrolyte membrane and a membrane electrode assembly which can be used for a fuel cell capable of operation under a nonhumidified state.
2. Description of the Related Art
Solid polymer type fuel cells having an electrolyte made of a solid polymer membrane have been known. The solid polymer type fuel cells use proton conductive polymer electrolyte membranes for their electrolytes. In general, a solid polymer type fuel cell includes an anode (fuel electrode), a cathode (oxidant electrode), and a polymer electrolyte membrane interposed between the anode and the cathode. The anode of the solid polymer type fuel cell is provided with a catalyst layer for promoting oxidation of the fuel. The cathode of the solid polymer type fuel cell is provided with a catalyst layer for promoting reduction of the oxidant.
The fuel to be supplied to the anode of the solid polymer type fuel cell is typically made of hydrogen, a hydrogen-containing gas, a mixed vapor of methanol and water, a methanol aqueous solution, or the like. The oxidant to be supplied to the cathode of the solid polymer type fuel cell is typically made of oxygen, an oxygen-containing gas, or air.
The polymer electrolyte membrane is typically made of sulfonate high-fluorinated polymer which has a backbone chain composed of fluorinated alkylene and a side chain composed of fluorinated vinyl ether having a sulfonic acid group at the end. This type of polymer electrolyte membrane exhibits ion conductivity sufficient for power generation when impregnated with an appropriate amount of water.
For this reason, the conventional solid polymer type fuel cells have required water management on their polymer electrolyte membranes, thereby causing to complicated and large-sized fuel cell systems.
To solve these problems ascribable to the water control on the polymer electrolyte membranes, a nonhumidified electrolyte membrane capable of conducting protons under a nonhumidified state has been developed as an alternative to the conventional polymer electrolyte membranes.
For example, Japanese Patent Laid-Open Publication No. Hei 11-503262 discloses a nonhumidified polymer electrolyte membrane which is made of such a material as polybenzimidazole doped with phosphoric acid.
When a polybenzimidazole or other basic polymer membrane is doped with phosphoric acid, the basic polymer membrane makes dimensional changes because of water absorption of the phosphoric acid. Moreover, when the phosphoric acid absorbs water, it comes out of the basic polymer membrane. As a result, the basic polymer membrane shrinks to cause wrinkles therein. Consequently, the storage of the produced basic polymer membrane and the fabrication of a membrane electrode assembly or a stack require special environmental facilities including a dry room. This gives rise to such problems as a manufacturing difficulty and an increased manufacturing cost.